Asynchronous Transfer Mode (ATM) is to become the base technology for the next generation of high speed networks. High speed networks support a great diversity of applications with different traffic and Quality Of Service (QoS) requirements. Such diversity require different control flow strategies. For example, certain applications like multimedia and time critical data applications require guaranteed levels of delay and throughput but can tolerate losses, while others like LAN traffic can tolerate variations in delay and throughput but are very loss sensitive.
In the ATM Reserved Bandwidth (RB) service, a user needs to establish a traffic contract with the network at call set-up before transmitting data. The contract includes specification of a desired QoS class and a set of traffic descriptors. The network, through resource allocation, provides the desired QoS for the ATM connection or refuses the call. The allocated bandwidth between a source and a destination may be less than the peak rate in order to benefit from statistical multiplexing gains, but it requires complex software and may cause traffic congestion and data losses.
In a LAN environment, sources are bursty and unpredictable. Traffic has an extremely high variability on time scales ranging over several orders of magnitude. For such unpredictable sources, the peak rate could be allocated in order to avoid heavy losses inside the network. However, the network would be inefficiently used, since bandwidth would be reserved even during idle periods. One way to increase link utilization for the network is through the addition of a Non Reserved Bandwidth (NRB) service class. In this service class, no bandwidth at all is reserved and the sources can transmit on a best effort basis, grabbing as much bandwidth among the remaining bandwidth as they can, without affecting the RB traffic. Thus, no resources being allocated in advance, the NRB service requires a control flow scheme in order to control the sources. The quality of the flow control mechanism qualifies the efficiency of the NRB service support in terms of cell loss ratio. The hop-by-hop congestion flow control mechanism disclosed in the European patent application EP94480125 implemented in the network nodes on the connections path is an improved flow control mechanism. With the selective (per connection) or global backpressure commands sent from an adjacent node, the entering traffic is selectively or globally resumed or throttled in each hop of the connection path.
However, NRB connections have no guarantee at all to be served by the ATM network, as discussed in the ATM Forum Technical committee in the traffic management working group, a need for a new service category is identified with the Available Bit Rate (ABR) service situated between the reserved and non reserved bandwidth services. For an ABR connection, the end-system shall specify to the network both a maximum required bandwidth (Peak Cell Rate, PCR) and a minimum required bandwidth (Minimum Cell Rate, MCR). The ABR service as defined at the ATM Forum includes also a flow control protocol between the source and the destination. The end-system is supposed to adapt its traffic in accordance with the feedback received from the flow control mechanism. The characteristics expected from ABR service are: guaranteed minimum bandwidth, a fair share of the available bandwidth and, with the flow control protocol, a low cell gloss ratio. As this service cannot control cell transfer delay, it is not intended to support real time applications. It applies to the same applications as NRB service particularly when the end-system requires a guaranteed QoS: critical data transfer (such as Defense information), super computer applications and any data telecommunication (as for NRB service : email, telex, fax, banking transactions, LAN interconnection etc . . . ) requiring better delay behavior (such as distributed file transfer, remote procedure call, computer process swap/paging).
While the cell loss ratio of the ABR service will depend on the quality of the flow control, the characteristics expected for the ABR service as for the guaranteed minimum bandwidth per connection and sharing fairly the available bandwidth, depend on the quality of the scheduling scheme. In the ATM network nodes of the industry, with this emerging ABR service, there is a need for providing a scheduling scheme able to support connections having a minimum bandwidth guaranteed and a fair share of the available bandwidth. This so called hereunder `Minimum Service` support, guarantees to each connection for which a minimum bandwidth has been required and accepted at connection set-up (MCR), to have this minimum bandwidth. Depending on the quality of the scheduling scheme implemented in the network nodes of the connection paths, the minimum bandwidth can be guaranteed either globally on the path or at any time and in each node of the network nodes. Both ways of guaranteeing a minimum bandwidth are compliant with the MCR parameter. The second characteristic of Minimum Service support is a fair sharing of the `remaining` bandwidth; this later comprises the bandwidth not reserved for the minimum service traffic and the part of the bandwidth reserved but not used by the minimum service traffic. In fact, if the scheduling scheme for support of Minimum Service can be combined with a network flow control mechanism such as the ABR flow control, the resulting support would improve the cell loss ratio; i.e., the more the network flow control the lower the cell loss ratio.
It is a first object of the invention to provide a scheduling scheme for support of Minimum Service connections such as ABR connections, guaranteeing the minimum usable bandwidth to each connection and a fair share of the remaining bandwidth between these connections.
It is a second object of the invention that the minimum bandwidth of each Minimum Service connection, if requested, is actually provided independently of the respective incoming rates of the other connections supported in the node. More particularly, the minimum bandwidth allocated to each Minimum Service connection is guaranteed even if the aggregate incoming traffic exceeds the output capacity.
It is a third object of the invention to support any mixed traffic comprising RB (CBR, VBR for the ATM Forum) or NRB (UBR for the ATM Forum) or Minimum Service connections such as the ABR connections defined at the ATM Forum, while maintaining the same characteristics or QoS expected for each type of service as if they where served alone; particularly, maintaining a low CLR for ABR service.